Support for multiparameter attribute assignment on virtual attribute writers

Currently, using multiparameter assignment on a non-column or
aggregation throws an error. Since the existing implementation used
the reflection to determine whether to create a Date or Time
instance to send to the writer, and there's no easy way to infer
the desired datatype from the method name or arity, I figured that
sending a Time instance for virtual attributes is best in all cases
(since a time inherently contains a date.)

The reasoning behind not generally resorting to composed_of was
done for the sake of consistency. Being that update_attributes
blindly sends to the attributes' writer methods, whether or not
said writer methods map to an actual DB column, it made sense that
special form helpers - i.e., date_select, should 'just work'.
Currently, due to the column reflection that happens in
#execute_callstack_for_multiparameter_attributes, only writer
methods that concretely map to a column are supported.

...I guess I just feel using #composed_of here and mapping the
(1i), (2i), etc. parameters is overkill, and all the form helper
methods should be able to support model writer methods using
#update_attributes.

Lastly, every Time contains a Date, and #foo_time= and
#foo_date= would 1) look silly and 2) tack on a lot more
foundational code. Otherwise, I don't see any other means to really
infer the desired format.

I'm just not sold that a default of Time makes sense. The
multi
parameter code handles more than just Dates and Times and you can
tell
it what the type is by specifying composed_of or something
similar.

I'd prefer to fix it 'right' rather than just default to time if
we're
unsure. Perhaps the best way to do this is to make sure that
#950
has
something simple for this case?